Ocean Stretching to the Horizon?
You need to reduce the focal length of your camera drastically, increase the size of 
the ocean, make the ripples more and smaller and flatten the angle of the camera
in relation to sea level.

EDIT (added): Take a close look at the original picture how imperfect it is! :) Why is your picture so perfect? :) :)
The human perception is trained to interpret subtle things like those imperfections. Often "perfect" renders are looking "sterilized" and "unnatural" (NO critism implied!!!).
Play with the depth of field of the camera and make it a little blurry in
the distance.
Then add some haze - just a little bit to obfuscate the "end of the horizon".
Infinity and long distances are often obfuscated and blocked for example in movies and the human brain is tricked into thinking "oh yeah...I can't what's there in the disctance...it must VERY FAR FAR away, because I can't see it!"